Diabetes is one of the most prevalent chronic diseases in the United States, and a leading cause of death, afflicting over 400 million diabetics in the world today. Estimates based on the 1993 National Health Interview Survey (NHIS) indicate that diabetes has been diagnosed in 1% of the U.S. population age <45 years, 6.2% of those age 45-64 years, and 10.4% of those age >65 years. As of 1995, an estimated 8 million persons in the United States were reported to have this chronic condition.
The total cost of diabetes in the United States has been estimated at $92 billion annually, including expenditures on medical products, hospitalization and the value of lost work. Substantial costs to both society and its citizens are incurred not only for direct costs of medical care for diabetes, but also for indirect costs, including lost productivity resulting from diabetes-related morbidity and premature mortality. Persons with diabetes are at risk for major complications, including diabetic ketoacidosis, end-stage renal disease, diabetic retinopathy and amputation. There are also a host of less directly related conditions, such as hypertension, heart disease, peripheral vascular disease and infections, for which persons with diabetes are at substantially increased risk.
Diabetes mellitus is a heterogeneous group of metabolic diseases which lead to chronic elevation of glucose in the blood (hyperglycemia). Diabetes is characterized by pancreatic islet destruction or dysfunction leading to loss of glucose regulation. The two major types of diabetes mellitus are Type I, also known as “insulin-dependent diabetes” (“IDDM”) or “juvenile-onset diabetes”, and Type II, also known as “non-insulin dependent” (“NIDDM”) or “maturity-onset diabetes”.
IDDM results from an autoimmune-mediated destruction of pancreatic β cells with consequent loss of insulin production, which results in hyperglycemia. Type I diabetics require insulin replacement therapy to ensure survival. While medications such as injectable insulin and oral hypoglycemics allow diabetics to live longer, diabetes remains the third major killer, after heart disease and cancer. However, these medications do not control blood sugar levels well enough to prevent swinging between high and low blood sugar levels, with resulting damage to the kidneys, eyes, and blood vessels. Data from the Diabetes Control and Complications Trial (DCCT) show that intensive control of blood glucose significantly delays complications of diabetes, such as retinopathy, nephropathy, and neuropathy, compared with conventional therapy consisting of one or two insulin injections per day. Intensive therapy in the DCCT included multiple injection of insulin three or more times per day or continuous subcutaneous insulin infusion (CSII) by external pump. Insulin pumps are one of a variety of alternative approaches to subcutaneous multiple daily injections (MDI) for approximating physiological replacement of insulin.
Type II diabetes is characterized by hyperglycemia in the presence of higher-than-normal levels of plasma insulin (hyperinsulinemia) and represents over 90% of all cases and occurs most often in overweight adults over 40 years of age. Progression of Type II diabetes is associated with increasing concentrations of blood glucose, coupled with a relative decrease in the rate of glucose-induced insulin secretion. In Type II diabetes, tissue processes which control carbohydrate metabolism are believed to have decreased sensitivity to insulin and therefore occurs not from a lack of insulin production, but a decreased sensitivity to increased glucose levels in the blood and an inability to respond by producing insulin. Alternatively, diabetes may result from various defects in the molecular machinery that mediate the action of insulin on its target cells, such as a lack of insulin receptors on their cell surfaces. Treatment of Type II diabetes therefore frequently does not require administration of insulin but may be based on diet and lifestyle changes, augmented by therapy with oral hypoglycemic agents such as, for example, sulfonylurea.
The endocrine portion of the pancreas is composed of the islets of Langerhans, which appear as rounded clusters of islet cells embedded within the exocrine pancreas. Four kinds of islet cells compose the endocrine portion of the pancreas: (1) alpha (α) cells, constituting 20% of islet cells, which secret glucagon, a hormone which raises blood sugar levels; (2) beta (β) cells, which secrete insulin, a hormone which lowers blood sugar levels; (3) delta (δ) cells, which secrete growth hormone inhibiting hormone (GHIH) or somatostatin, a hormone which inhibits the secretion of insulin and glucagon; and (4) φ cells, or pancreatic polypeptide (PP) cells, which synthesize pancreatic polypeptide. Glucagon acts on several tissues to make energy available in the intervals between eating. In the liver, glucagon causes breakdown of glycogen and promotes gluconeogenesis from amino acid precursors. Pancreatic polypeptide inhibits pancreatic exocrine secretion of bicarbonate and enzymes, causes relaxation of the gallbladder, and decreases bile secretion. Insulin is known to cause the storage of excess nutrients arising during and shortly after eating. The major target organs for insulin are the liver, muscle and fat-organs specialized for storage of energy.
The most abundant cell in the islets, constituting 60-80% of the cells, is the insulin-producing β cell. The β cells of the human fetal pancreas are different from adult pancreatic β cells in that they release little or no insulin in response to glucose. (See, e.g., Tuch, B. E. et al. (1992) J. Endocrin. 132:159-67). This has been observed in both humans and rodents, and resembles the delayed insulin response to glucose observed in patients with Type II diabetes or malignant insulinoma. (Hellerström and Swenne (1991) Diabetes 40(2):89-93; Tuch et al., supra). The inability of fetal β cells to produce insulin in response to glucose is not believed to be due to an inability to process insulin precursors. Adult human β cells synthesize preproinsulin and convert this into proinsulin (hPI) in the endoplasmic reticulum. Thereafter, hPI is split into insulin and C-peptide via a regulated pathway in the secretory granules. The rate of conversion of hPI in the adult β cell is high, resulting in a low hPI:insulin ratio both as regards to content and secretion (Gold, et al. (1981) Diabetes 30:77-82). This is also observed for fetal β cells, suggesting that β cell immaturity is not due to differences in the storage and release of proinsulin. (Tuch et al., supra). The acute release of both hPI and insulin from the fetal β cell in response to an increase in Ca2+ and cAMP suggests that the cell releases its secretory products via a regulated, rather than a constitutive pathway. (Rhodes and Halban (1987) J. Cell Biol. 105:145-53).
The lack of glucose responsiveness in fetal β cells is thought to be due to immature glucose metabolism. The molecular mechanism underlying glucose-induced insulin secretion in adult β cells involves the closure of ATP-sensitive K+ (KATP) channels in the plasma membrane, thereby inhibiting K+ efflux through K+ATP channels, leading to depolarization of the cell membrane. (Jones, P. M. and Persaud, S. J. (1998) Endocrine Reviews 19(4):429-61; Mendonca et al., supra; Cook, D. L. and Hales C. N. (1984) Nature (London) 311:271-73). Consequently, cytosolic Ca2+ concentration increases as a result of the membrane depolarization and Ca2+ influx through L-type (voltage-sensitive) Ca2+ channels. Glucose raises the intracellular concentration of cAMP and of regulators derived from membrane phospholipids, including inositol triphosphate (IP3), diacylglycerols (DAG), arachidonic acid (AA) and phosphatidic acid. (See Jones and Persaud, supra). It has been suggested that reduced insulin secretion in response to glucose reflects the uncoupling between glucose metabolism and membrane cell depolarization. (Mendonca et al., supra). Studies indicate that the ATP-sensitive K+ channel, although fully developed, is not properly regulated in the fetal β cell and that the deficient secretory response to glucose may reflect an immature mitochondrial glucose metabolism resulting in an inability to close the otherwise normal ATP-sensitive K+ channel. (Hellerström and Swenne, supra.).
Pancreatic development occurs in discrete stages and is regulated by endocrine hormones produced by pancreatic cells themselves or by other tissues. In the rat, the pancreatic anlage forms at embryonic (“e”) day 10.5 (“e10.5”) by fusion of the dorsal and ventral pancreatic primordial buds that arise as protrusions from the duodenal endoderm. (Pictet, R. and Rutter, W. J. (1972) “Development of the Embryonic Endocrine Pancreas.” In D. Steiner and N. Freinkel (eds.) Handbook of Physiology, The Endocrine Pancreas, Vol. 1, Section 8, Am. Physiol. Soc., pp. 25-66; Myrsén-Axcrona, U. et al. (1997) Regulatory Peptides 68:165-75). Islet hormones appear sequentially in the developing pancreas: for example, glucagon appears at e10 in mouse and e11 in rat, insulin producing cells appear in e12, somatostatin producing cells appear at e17. (See Myrsén-Axcrona et al., supra). It is thought that pancreatic islet cells differentiate in response to endocrine signals from a common precursor cell in the pancreatic ducts. Sometime between the end of the rat fetal stage (e21) and neonatal stages (post-birth) the fetal β cells acquire the ability to secrete insulin in response to glucose. The insulin response at this age is monophasic and is not blocked by Ca2+ antagonists. A clear biphasic pattern of insulin secretion in response to glucose is detected only 3 days after birth. (Mendonca, A. C. et al. (1998) Brazilian J. Med. Biol. Res. 31(6):841-46). The mechanism by which this “gain of function” or “gain of glucose responsivity” is achieved is not known, nor have the factors that regulate the maturation and gain of function been identified or characterized. In addition, the physiological changes associated with gain of glucose responsivity in pancreatic β cells are not known.
The instant invention is based on the discovery that a factor, “peptide yY” or “PYY”, triggers gain of function in glucose non-responsive fetal and adult islets which leads to glucose responsivity, and therefore provides therapies for diseases affecting glucose metabolism such as Type II diabetes.